Figure 2.5 Time 3 hours

The embryos continue to undergo rapid cell division, the number of cleavages is difficult to estimate by this stage. It is increasingly difficult to distinguish between a fertilised and unfertilised egg. The edges of the eggs have a corrugated appearance and it is difficult to visualise individual cells.

Figure 2.6 Time 4 hours

There appears to be very little difference in external appearance of the embryo even though cell division continues at a rapid rate. At this time a secondary envelope, within the external hatching envelope, can be seen. An invagination can be seen on the surface of the egg that is due a process called gastrulation. It involves a dynamic morphological change from a monolayered to a multilayered embryo, accompanied by tissue differentiation.